I’m going to put the rotary A axis stepper motor on a plug and socket at to the control cabinet. This will allow the chuck and gear box to be conveniently removed from the bed when it is not actually required. The connector needs to be polarized and rated at about 3 A with four terminations and perhaps ground. I may have something in my junk box that will meet this duty.
I couldn’t find a suitable plug and socket for the rotary axis in my spare parts collection so I spent a few dollars and purchased new, including a blanking cap for the socket. The socket is rock sold. The plug and socket are way over-rated at 16 Amps 240 V. They have an IP 68 rating which is good for cutting fluid. The cable mount is a compression fit to the cable cover onto the outer insulation. The plug cannot be pulled off the cable and puts no strain on the terminations. Before selecting the mounting position I made sure that the made sure that the plug and cable would sit clear of the maximum traverse of the bed.
Making the holes for remounting the heat sink with the machine on the bench was a bit of a challenge. The 22 mm diameter hole for the rotary axis socket was drilled with a Dremel flexible drive, then taper reamed, and finished with a chassis punch.
Note: the control cabinet electrical and electronic wiring is on a separate steel base from the cabinet proper. I suspect that the intention of this arrangement is to allow external assembly of the control cabinet, provide clean mounting mounting of the cabinet against the column, and perhaps to reduce vibration to the electronics. In my opinion a better design would use DIN rails or equivalent to reduce weight, save material, provide assembly and service and access, and allow for modifications. The electronics should easily handle long term machine vibration.
Figure 1. 22 mm Hole Through Cabinet and Electronics Chassis
(Yes, I cleaned this up before mounting the rotary axis socket.)
Figure 2. IP68 16 Amp Polarized Rotary Axis Plug and Socket Fitted
(The plug and cable have clearance from the bed under all traverses.)
When I ordered the CNC mill I also specified provision of a fourth rotary axis. This arrived with the machine, but without a tail stock. After contemplating designing and making a tail stock from scratch it will be cheaper quicker and easier to purchase one specifically made for the rotary axis. The new tail stock is on order at a cost of US$40 plus expedited freight. If you order a fourth rotary axis make sure that you also order the tail stock otherwise you will not be able to fully utilize the rotary axis.
The new tail stock arrived today. It was nicely packaged and arrived in just a few days. It is probably functional but the quality is best described as nasty (sometimes you get exactly what you pay for). The finishing is particularly poor. Many holes are not deburred and finishing on machined surfaces is non-existant. The spindle clamp is not perpendicular to the tail stock body and hand wheel thread is incredibly loose in the end plate. Both of these could also be improved with brass pressure plates.
Figure 3. Tail Stock
The biggest problem with the tail stock is that the centre height is wrong. When I ordered this part it was supposed to be compatible with the 4th rotary axis supplied with the XK7113D. Clearly it is not. WMT have got this wrong. Their recommended solution is to pack up the rotary axis. While this will work, it shouldn’t be required at all.
Figure 4. Centre Height Mismatch - Oops!
Rather than pack up the rotary axis the aluminium extrusions that support the sleeve barrel can be milled down by approximately 15 mm to get the centre on the axis of the chuck. The clamp female thread will need to be over drilled and either an insert fitted or re-tapped with a new (larger) clamp lever. And the hand-wheel end plate will need to be re-made. The spindle has a fixed live centre. I’ll be remaking this too with a Morse taper.
Thankfully I don’t need the tail stock immediately - all of this can wait.
I’ve tested the rotary axis and it works under Mach3 control, but there is an entire degree of backlash in the gear box! At the radius of the chuck (~40 mm) this allows ~0.7 mm of free vertical movement. This totally defeats the resolution of the rotary axis stepper motor drive and makes it impossible to machine true flat surfaces under numerical control about an axis. It is also likely to impart tool chatter onto machined surfaces near the axis.
There are other rotary axes available from other suppliers and all of these use direct or coaxial drives with notched belts. The reason for this is to avoid the backlash inherent in worm and wheel gear drives.
The backlash has been confirmed by WMT. They suggest compensating for it in the model. I’m not certain if this work for the reasons stated above.
On further inspection the gear box is actually shimmed on a collection of washers off the base plate. There is no way this can create a positive, secure and axially parallel mount for the chuck. The reason this has been done is to provide clearance between the chuck and the bed mounting plate. This is poor design.
Figure 5. Particularly Nasty Packing for Chuck Clearance
The bearings on the chuck shaft are plain roller bearings. They should be taper bearings to resist axial loads. This implies that this worm and wheel gear box was never intended for this duty, as do the fins on the casing. These are designed to dissipate heat implying relatively high speed use, say in a winch drive.
So the rotary axis and tail stock are simply not up to the specifications of the XK7113D CNC mill. WMT would be better to source compatible accesses with appropriate accuracy. I have contacted WMT requesting to return these accessories for a refund. They have yet to reply to this request.
Once the mill is actually cutting metal effectively I may be able to re-use the chuck and stepper motor to make my own timing belt driven rotary axis. Research suggests that, subject to load, these have almost zero backlash. There will be some machining to do for the bearing housing and the timing belt pulleys. And I’ll need to purchase a timing belt and two tapered bearings.
This is on hold for now.
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